janssonrpcc: do not lock in mod destroy

- all children are killed already and if lock was already set results in
  a deadlock

(cherry picked from commit 185db312ed91bba6c997a42a84c4da1e4ce2319f)

src/modules/janssonrpcc/janssonrpcc_mod.c

@@ -215,7 +215,6 @@ static int child_init(int rank)
 void mod_destroy(void)
 {
 	if(jsonrpc_server_group_lock) {
-		lock_get(jsonrpc_server_group_lock); /* blocking */
 		lock_dealloc(jsonrpc_server_group_lock);
 	}